The Influence Of Combustion Boundary Conditions On Combustion And Emission For Automotive Heavy-duty Compressed Natural Gas Engine

World attention on energy crisis and environmental pollution leads a quest for energyconversation and clean burning fuels in the internal combustion engines. Compressed naturalgas （CNG）, regarded as one of the most promising alternative fuels, is widely used inautomotive engines due to its rich resource and friendly emissions. Howerver, flamepropagation velocity is slow during the combustion process in engines when fueled with CNGdue to its physiochemical properties, which restricts the development of high efficiency andultralow emissions. In addition, the reasonable in-engine emission control technologies shouldbe proposed to meet tightening emission regulations due to the unavailability of Three WayCatalysts （TWC） on lean-burn CNG engine.In order to solve those problems, the in-cylinder flows and the distribution of mixtureconcentration as well as combustion process were investigated in the electronic controlled andmulti-point injection lean-burn CNG engine by means of experiment and3D numericalsimulation. The physical and chemical factors were analyzed which influence the flamepropagation velocity of CNG engine. The influence of various lean-burn modes on workingprocess of CNG engine has been comparatively studied. Based on these researches, advancedin-cylinder flows and mixture concentration distribution as well as combustion mode wereproposed to support the development of high efficiency and cleaning burning CNG engine.For the reasonable combustion boundary conditions, the influences of ignition timing andinjection timing on combustion process and emission characteristic were experimentalinvestigated firstly. Combine the analysis of numerical simulation results in the injection timingresearch, which could visually explain the experimental burning tendency. The research resultsshowed that: within certain limits, the constant volume ratio increased during combustion whenthe ignition timing was ahead and the injection timing was behind, which could obtain moreworking power so that the fuel economy and NOx emission were increased. The laggedinjection timing changed the fuel concentration distribution in the direction of cylinder axisfrom “up thin down thick” to “up thick down thin”. The various optimal control strategies basedon NOx emission or fuel economy could be realized by adjusting ignition and injection timing.The European Stationary Cycle13（ESC13） results satisfied China stage4emission regulationsbased on the NOx emission optimal control strategy.In order to solve the problem of slow flame propagation in the CNG engine, the enhancedturbulence intensity during the late period of compression stroke was required to promoteburning velocity. The evolution of in-cylinder flows was described by3D computational fluid dynamic tool STAR-CD, and the methods based on changed shape of combustion chamber andfuel injection location were proposed to enhance the turbulent kinetic energy （TKE）. The crossand vertical sections coupled with statistics results of critical area sampling revealedcharacteristics of in-cylinder flows, like velocity vectors, TKE and distribution of mixtureconcentration. The influence of in-cylinder flows on combustion process and emissions wasstudied combined with experimental results. The study results showed that:1. The shorter phase position distance between the peak TKE and the peak heat release rateduring the late period of compression stroke was more beneficial to the enhanced burningconstant volume ratio and then increased thermal efficiency, which the distance of crosscombustion chamber was10.5°CA shorter than that of cylindrical combustion chamber.2. The analysis of in-cylinder microphenomenon indicated that the original cylindricalcombustion chamber maintains high speed large scale vortex motion so that the horizontalvortex ratio is31.7%higher than that of cross combustion chamber. On the contrary, the crosscombustion chamber sheared the large scale vortex motion into micro scale turbulence, its peakTKE was78.2%higher than that of original combustion chamber. According to the statisticresults of sampling points on the ribs circle radius, during the combustion process, even thoughthe average velocity of cross combustion chamber flows was lower than that of cylindricalcombustion chamber, the velocity was more disordered so that the velocity standard deviationwas larger, which existed more turbulence.3. The numerical and experimental results indicated that large TKE was disadvantage tothe early flame development under the same mixture concentration, which led to a longerignition delay period. However, the large TKE was required during the main combustion periodto form rapid burning. As a result, the large TKE in the cross combustion chamber acceleratedthe burning velocity, enhanced the combustion quality and then increased the fuel economy5.2%higher than that of the cylindrical combustion chamber at WOT as well as the23.7℃decreased discharge temperature. The ESC13results showed that the NOx emission of crosscombustion chamber was40%higher than that of the cylindrical combustion chamber, whereasthe HC and CO emissions were decreased.4. Studies on the4kinds of fuel injection locations, the results showed that the influence offuel injection location on the average TKE was unobvious, but the local flow state was changed.The fuel injection location changed the distribution of mixture concentration in the radialdirection. The reasonable distribution of TKE and mixture concentration could be formedaround the spark plug. The experimental combustion and emission results efficiently validatedthe numerical flow and mixture concentration analysis. The potential of various dilution methods to control NOx emission of CNG engine wasinvestigated experimentally by engine internal purification. Three kinds of dilution methodswere stoichiometric with Exhaust Gas Recirculation （EGR） dilution combution, air dilutioncombustion and twofold dilution combustion that combined extra air and EGR. The influenceof them on combustion process, emission characteristic and lean-burn limitation on CNGengine was researched, the results showed that:1. In the combustion method of stoichiometric with EGR, based on the optimal ignitiontiming and injection timing as well as EGR rate, the NOx emission could decreased by90.2%.However, the contribution of EGR on decreased exhaust temperature was limited whilestoichiometric combustion. Compared with twofold dilution method, its exhaust temperaturewas9.7%higher while the average EGR rate was21%, yet the average EGR rate was6.5%inthe two-fold dilution.2. In the combustion method of air dilution, the increased extra air amount led to decreasedin-cylinder combustion pressure and heat release rate, and to lagging corresponding phasepositions as well as the increased coefficient of cyclic variation of the peak combustion pressure.The NOx emission reached their peak between the excess air coefficient named λ from1.1to1.2, and then the increased λ resulted in the sharp decreased NOx emission, increased HCemission and reduced CNG consumption. Compared with the twofold dilution method, thedilution limit of the air dilution method was narrow so that unstable combustion could appear.The NOx emission of twofold dilution method was separately43.2%,47.1%and50.9%lowerthan that of air dilution method under the ESC13speeds while the CNG consumptions wereequal.3. Compared the ESC13test emission results of that three kinds of dilution methodsindicated that, stoichiometric with EGR could obtain the optimal NOx emission and the airdilution method could obtain the optimal fuel economy while the twofold dilution combined theadvantages of those two methods so that obtained the optimal NOx-be relationship. The NOxemission could meet China stage4and5emission regulations through these three kinds ofdilution methods while the combustion boundary conditions were optimized. Moreover, thetwofold dilution method had the optimal fuel economy.In order to utilize the great ability of EGR to control NOx emission and to alleviate thedisadvantage in burning velocity, to realize the further decreased of NOx emission whilemaintained the thermal efficiency, the tratified EGR was proposed. For decoupling theinteraction of traditional EGR loop and CNG engine, avoiding the complex control on theamount and introduced pressure, and sloving the large differential pressure between traditional EGR and fuel gas rail, the CO₂was adopted to replace the EGR. The traditional EGR is equal toair blending CO₂and the stratified EGR is equal to CNG blending CO₂. The influence of thesetwo CO₂blending methods on the mixture formation and mixture formation as well as emissionwas numerical and experimentral investigated, and the results indicated that:1. The CO₂concentration distribution of CNG blending method was the same as the CH4concentration distribution. Compared with air blending CO₂, the peak of total dilution rate inthe CNG blending CO₂method was smaller, which realized the ideal distribution of that thickregion with large CO₂and thin region with less CO₂.2. The experimental results indicated that the increased CO₂delayed the burning velocityand CA50, increased the ignition delay period, combustion duration and cyclical variation. TheCNG blending CO₂has weaker influence on the burning velocity. The optimal be-NOxrelationships of those three CO₂blending methods （no blending, air blending and CNGblending） were obtained under the optimized combustion boundary conditions at1750r/min-75%load. Blending CO₂could get better be-NOx relationship than that of no CO₂blending method. CNG blending CO₂obtained the optimal be-NOx relationship region than theothers. It improved trade-off between NOx and be, which was advantaged to the high efficiencycleaning combustion of the heavy-duty lean-burn CNG engine.